The Flowering Pathway in Wheat - Wheat Science and Trade - page 62

Heading Date 1 ( HD1 ) is an ortholog of CO (Yano
et al., 2000), but HD1 acts in an opposite way,
repressing FT and fl owering in rice, compared
with CO in Arabidopsis (Kojima et al., 2002). The
interaction of HD1 with FT is altered such that
FT expression is inhibited under LD conditions
(Hayama et al., 2003; Hayama and Coupland
2004).
Among these three species, two related genes
AP1 and FT appear to play the same role by
promoting fl owering. However, unlike wheat,
Arabidopsis or rice has not shown allelic varia-
tion at AP1 or FT , with no subsequent varia-
tion in responses to vernalization. The wheat
VRN-2 gene has no clear orthologues in Arabi-
dopsis or rice, and the Arabidopsis FLC has no
clear orthologues in rice or wheat, but FLC
and VRN-2 have an analogous function to the
rice HD1 that represses fl owering under LD.
These observations suggest that these three
species have relatively conserved genes but with
diverse functions ( AP1 , FT ) or have different

genes but with similar functions ( VRN-2 , FLC ,
HD1 ).
Barley shows the same responses to both ver-
nalization and photoperiod as wheat. Its diploid
genome, wealthy genetic sources, and conserved
gene structure and function have provided trans-
ferable information for understanding the fl ower-
ing pathway in wheat (Danyluk 2003; Trevaskis
et al., 2003, 2007; von Zitzewitz et al., 2005;
Yan et al., 2005, 2006; Dubcovsky et al., 2006;
Sz ″ cs et al., 2007).

A model for the wheat
fl owering pathway
Various models for the wheat fl owering pathway
have been proposed (Yan et al., 2003, 2006;
Loukoianov et al., 2005; Dubcovsky et al., 2006;
Kane et al., 2007; Trevaskis et al., 2007). Current
understanding of wheat fl owering time genes
yields a model for the wheat fl owering pathway as
updated in Fig. 3.1.

Winter wheat Spring wheat

Vernalization
Long days

Short days

Vrn-3

vrn-3 ( FT )

vrn-2

TaVRT2

Vrn-2

vrn-1 ( AP1 )

Vrn-1

CArG

Undifferentiated apex Flowering

Transition

Short days
Low temperature

Long days (PPD-1)
Favorable temperature

Fig. 3.1 ” indicates repres-

sion. Winter wheat has a genotype consisting of dominant Vrn2 and recessive vrn1 and vrn3 alleles (left side); a mutation for
any of them will result in a spring wheat genotype (right side). In winter wheat, vrn1 is up-regulated by vernalization (under
long days, LD) by releasing a repressor TaVRT2 that binds to the vrn1 promoter. This recessive vrn1 is also activated by vrn3 ,
which is up-regulated by vernalization. The Vrn2 allele is down-regulated by short days (SD) or vernalization, and the repres-
sion on Vrn2 is released by vrn1 during vernalization. Long days via PPD-1 and moderately high temperature are required
for photoperiod-sensitive cultivars of spring wheat and winter wheat in which any requirement for vernalization has been
satisfi ed to accelerate fl owering.

An updated model of the wheat fl owering time pathway: Thin arrows indicate promotion and “